Speedometer Question

syenisch · 12-29-2008, 08:00 PM

No, not the usual "how far off is the speedometer?" question!

Got a weirder one here for you...

How is the vehicle's speed determined? I'm assuming it's off the rate of revolution of the tires, but which ones - front or rear?

ZIPNBYE · 12-29-2008, 08:03 PM

Typically it's the driven axle that's metered.

imdanomite · 12-29-2008, 08:06 PM

rear wheels

MOCKBA · 12-29-2008, 11:17 PM

I use front wheel on my bicycle since rear can slip.

jeremyc74 · 12-30-2008, 12:43 PM

The speed data isn't coming from the front wheels for certain. When you spin the back tires the speedo moves even if you're sitting still.

Where exactly they're pulling the signal from is hard to say. In the past the speedometer/OD information came from a sensor (or cable if you want to go back a few decades) on the output shaft of the transmission. That may still be the case, but on modern cars the ABS/Stability control systems require wheel speed to operate anyway, so that's another source that could be used.

I'm guessing it's still taken off the output shaft of the transmission though.

12-29-2008, 08:00 PM	#1
syenisch Captain 111 Rep 810 Posts Drives: 2015 435i Gran Coupe Join Date: Jun 2008 Location: Winter Garden, FL iTrader: (1)	Speedometer Question No, not the usual "how far off is the speedometer?" question! Got a weirder one here for you... How is the vehicle's speed determined? I'm assuming it's off the rate of revolution of the tires, but which ones - front or rear?
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12-29-2008, 08:03 PM	#2
ZIPNBYE Let's save military ranks for those who earn it! 45 Rep 1,161 Posts Drives: ...on the Ragged Edge Join Date: Dec 2008 Location: Edmonton, AB, Canada iTrader: (6) Garage List 2013 BMW e92 M3 Cou ... [7.50] 2013 BMW e84 X1 xDr ... [7.00]	Typically it's the driven axle that's metered. __________________ Richard 2013 BMW M3 Individual -- Fire Orange (sold) 2013 BMW X1 xDrive35i M-Sport -- Valencia Orange (sold) 2011 BMW 1-series M Coupe (#205/220) -- Valencia Orange (sold) 2009 BMW 135i M-Sport Coupe -- Space Grey (sold)
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12-29-2008, 08:06 PM	#3
imdanomite likes boobies ( o )( o ) 13 Rep 275 Posts Drives: 2009 BMW 135i Join Date: Jan 2009 Location: MD iTrader: (0)	rear wheels
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12-29-2008, 11:17 PM	#4
MOCKBA Captain 54 Rep 870 Posts Drives: 135 convertible Join Date: Nov 2006 Location: SFBA iTrader: (0)	I use front wheel on my bicycle since rear can slip. __________________ 535i/HUD Kamerton brings me true sound fidelity
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12-30-2008, 12:43 PM	#5
jeremyc74 Banned 76 Rep 5,970 Posts Drives: '08 135i Montego/Terra Join Date: Feb 2008 Location: Evansville, IN iTrader: (0)	The speed data isn't coming from the front wheels for certain. When you spin the back tires the speedo moves even if you're sitting still. Where exactly they're pulling the signal from is hard to say. In the past the speedometer/OD information came from a sensor (or cable if you want to go back a few decades) on the output shaft of the transmission. That may still be the case, but on modern cars the ABS/Stability control systems require wheel speed to operate anyway, so that's another source that could be used. I'm guessing it's still taken off the output shaft of the transmission though.
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